In recent years, pursuant to the higher integration of very-large-scale integrated circuits (VLSI), studies are being conducted for using materials having lower electrical resistivity as the electrode material or the wiring material. Under the foregoing circumstances, high-purity tungsten having low resistivity and thermal and chemical stability is being used as the electrode material or the wiring material.
The foregoing electrode material or wiring material for VLSI is generally produced by way of the sputtering method or the CVD method, but the sputtering method is being widely used in comparison to the CVD method since the structure and operation of the sputtering device are relatively simple, deposition can be performed easily, and the process is of low cost.
Nevertheless, a tungsten target that is used for the deposition of the electrode material or wiring material for VLSI in the sputtering method is required to be of a relatively large size of φ 300 mm or larger, and to have high purity and high density.
Conventionally, as methods of preparing this kind of large-size tungsten target, the following methods are known; namely, a method of preparing an ingot by way of electron beam melting and subjecting the obtained ingot to hot rolling (Patent Document 1), a method of subjecting tungsten powder to pressure sintering and thereafter to rolling (Patent Document 2), and a so called CVD-W method of laminating a tungsten layer on the entire surface of a tungsten bottom plate by way of the CVD method (Patent Document 3).
Nevertheless, with the method of rolling the ingot obtained based on electron beam melting or the sintered compact obtained by subjecting tungsten powder to pressure sintering, there are problems in that the target is mechanically fragile since the crystal grains easily coarsen, and granular defects referred to as particles are easily generated on the sputtered film. Moreover, although the CVD-W method yields favorable sputtering characteristics, there is a problem in that it is extremely time-consuming and expensive to prepare the target.
In addition, disclosed is technology of using tungsten powder containing 2 to 20 ppm of phosphorus (P) as the raw material, and sintering this raw material by way of hot pressing and HIP in order to produce a tungsten target having an average grain size of φ 40 μm or less (refer to Patent Document 4).
In the foregoing case, the requirement is the inclusion of phosphorus in an amount of 2 ppm or more, but the inclusion of phosphorus caused a problem of deteriorating the grain boundary intensity of the sintered compact. In particular, if it is a large-size tungsten target and large amounts of phosphorus are contained therein, abnormal grain growth tends to occur locally, and grains of approximately 500 μm to 2 mm will be scattered about. Crystals that were subject to the foregoing abnormal grain growth will further deteriorate the grain boundary intensity, and there is a problem in that chipping will occur during the machining process for grinding the target and the product yield will deteriorate.
Although it is possible to devise the sintering conditions for resolving the problem of the abnormal grain growth of tungsten, there is a problem in that this merely results in a more complex production process and does not offer a solution for stable production.
In addition, disclosed is technology of acquiring a high-purity tungsten target having a purity level of 3N5 to 7N and an average grain size of 30 μm (refer to Patent Document 5). Nevertheless, in the foregoing case, the total impurity content and the impurities which are undesirable in semiconductors (Fe, Cr, Ni, Na, K, U, Th, etc.) are merely prescribed, and there is no disclosure regarding the problems caused by the inclusion of phosphorus.
Accordingly, this technology has numerous problems; specifically, occurrence of defective targets, deterioration of yield in the target production process, increase in production costs, and so on.
Under the foregoing circumstances, Patent Document 6 developed by the present Applicant (“Nippon Mining Co.” as the Applicant prior to the name change) is the most effective method for producing a high-purity tungsten powder. For example, ammonium metatungstate is dissolved in water to create a tungsten-containing aqueous solution; inorganic acid is added to the tungsten-containing aqueous solution; the solution is heated to precipitate tungstate crystals; after performing solid-liquid separation, the tungstate crystals are dissolved in ammonia water to create a purified mother water for ammonium paratungstate crystal precipitation and an undissolved residue containing impurities such as iron; the undissolved residue is subject to separation cleaning; the purified mother water for ammonium paratungstate crystal precipitation is heated; and inorganic acid is added to adjust the pH for precipitating the ammonium paratungstate crystals; whereby high purity ammonium paratungstate crystals are produced.
The ammonium paratungstate crystals obtained with the foregoing method are further calcined to form a tungsten oxide, and hydrogen reduction is additionally performed at high temperature in order to obtain a high-purity tungsten powder. In many respects, Patent Document 6 is the fundamental technology upon producing a high-purity tungsten powder, but it was necessary to make additional improvements for further reducing the phosphorus content under the present conditions where the reduction of the phosphorus content are strongly required.    [Patent Document 1] Japanese Laid-Open Patent Publication No. S61-107728    [Patent Document 2] Japanese Laid-Open Patent Publication No. H3-150356    [Patent Document 3] Japanese Laid-Open Patent Publication No. H6-158300    [Patent Document 4] Japanese Laid-Open Patent Publication No. 2005-307235    [Patent Document 5] WO2005/73418    [Patent Document 6] Japanese Laid-Open Patent Publication No. H1-172226